Wellbores are generally drilled using a drilling string formed of a number of drill pipes connected end to end which extends from the surface to a bottom hole assembly (BHA) at its terminal end. The bottom hole assembly (BHA) in an oil well drilling string typically consists of a drill bit at the bottom, and above that a motor and power section. The power section is essentially a turbine that extracts power from the flow of drilling mud pumped from the surface and rotates the drill bit. Above the power section there are typically a number of heavy drill collars that add mass to the bottom hole assembly. These contain a central bore to allow the flow of drilling mud through to the power section. The wellbore is drilled by the BHA in order to reach a subterranean formation of interest which may then be assessed, for example to determine whether hydrocarbons may be present in the formation.
Initially, wellbores were drilled without any form of directional monitoring while drilling. Instead, sections of wells were surveyed after they had been drilled, by which time they could easily have deviated significantly from their intended path. To address this problem, Measurement While Drilling (MWD) equipment was introduced using accelerometers and magnetometers to determine the orientation of the drill string during drilling. This information could be conveyed to the surface in real time, usually in the form of pressure pulses in the drilling mud column pumped from the surface.
MWD equipment is typically contained in a small diameter probe assembly that sits within a drill collar such that an annular space exists between the probe assembly and the drill collar to allow the passage of drilling mud around the probe assembly and down to the power section. In some examples, the probe assembly is supported within the drill collar with centralisers at the base of the probe assembly and higher up. The centralisers usually consist of rubber fins or metal bow springs and support the probe assembly such that an annular space exists between the probe assembly and the drill collar. In other examples, the probe assembly is removably seated within a sleeve which is fixed inside the drill collar. For example, the probe assembly may be supported within the sleeve of a mule shoe held inside the collar. Typically, the probe assembly is seated in its support such that it is held to a specific rotation but is not otherwise fixed relative to the drill collar. This allows the probe assembly to be removed from the BHA by lowering a cable assembly down the inside of the drill pipe and collars, attaching it to the top of the probe and hoisting it back to the surface. This operation may be performed, for example to replace batteries or faulty equipment in the probe, without the need to remove the BHA, collars and all the drill pipe from the well, which is a very time-consuming process. Once the batteries or faulty equipment have been replaced, the probe assembly may be lowered back into the BHA and drilling may recommence. This retrievability and reseatability is viewed in the industry as very desirable.
In addition to the presence of MWD equipment in the probe assembly to determine the orientation of the drill string, additional sensors, such as natural gamma ray sensors and shock and vibration monitors, may also be included in the probe assembly and their data included in the data stream sent to the surface. These sensors may allow measurements relating to the properties of a formation to be transmitted to the surface while drilling is taking place, or in “real-time”. Such Logging While Drilling (LWD) equipment allows measurement results to be obtained before drilling fluids invade the formation deeply and may allow measurements to be obtained from the formation in the event that subsequent wireline operations are not possible.
However, the probe assembly is not the ideal location for all sensors. Some sensors, such as bore pressure sensors and formation resistivity sensors, need access to the borehole surrounding the drill collar and, therefore, must be mounted on an outer surface of a drill collar. Communicating with such tools has presented the industry with something of a challenge. Some attempts at solving this problem have involved securely bolting the probe assembly to the inside surface of the collar to allow physical and pressure sealed connection between the collar-mounted sensor and the probe assembly through an aperture in the collar. However, this results in the loss of retrievability and reseatiblity of the probe assembly independent of the drill collar.
Accordingly, it would be desirable to provide a subassembly for the bottom hole assembly of a drill string having a probe assembly and with which data can be easily transferred between a sensor and the probe assembly without compromising the retrievability of the probe assembly.